Multibaseline NenuFAR Interferometry in Transit Mode: Continuous Low-frequency Spectrum of the Crab Nebula

Abstract The Crab Nebula, a historical supernova remnant originating from the explosion recorded in 1054 CE, remains a cornerstone of multiwavelength astrophysical research. Its evolution across the radio, optical, and X-ray domains continues to be studied extensively. At low radio frequencies (&lt;100 MHz), accurate spectral measurements are hindered by strong scintillation effects from the compact central component. However, under favorable space weather and ionospheric conditions, reliable observations of the integrated spectrum become feasible. We present new low-frequency measurements of the Crab Nebula’s integrated radio spectrum, obtained in 2025 using the New Extension in Nançay Upgrading LOFAR (NenuFAR) radio telescope, operating over the range 22–75 MHz. No significant flux decrease is detected between 1963 and 2025 within observational uncertainties. The full low-frequency spectral profile comprises two components: a compact source within the nebula and an extended shell contribution. The shell’s flux density spectrum peaks at 2069 ± 200 Jy near 27.5 ± 0.5 MHz, consistent with a turnover caused by unshocked ejecta. Its spectral index between 50 and 70 MHz is ∼−0.36, following the convention S ν ∝ ν α , while the total spectrum follows a power law with index ∼−0.54. Free–free absorption modeling yields internal parameters of T ≈ 1000 K, ionization state Z ≈ 3.3, and emission measure EM ≈ 38 pc cm −6 . The inferred electron density of the unshocked ejecta is ∼15 cm −3 , corresponding to a mass of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>1.8</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.30</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.46</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:math> . These results are complemented by URAN–2 observations at 10.7, 16.7, 20.0, and 25.0 MHz.